This invention relates to improvements in self-threading fasteners and to roll threading dies used for making such fasteners.
The type of fastener with which the present invention is concerned is one having a cross section in the form arcuate lobular polygon having arcuate sides with intermediate arcuate lobe that form the apices of the polygon. Generally speaking, the polygon has an odd number of lobes and in particular is of arcuate triangular configuration, namely having three equally spaced lobes each turn of the thread. Screws of this type have enjoyed enormous commercial success. Such screws are roll threaded on essentially conventional dies which form the thread on a screw blank of lobular cross section.
Because the screw blank must be of lobular cross section it is necessary to provide suitable tooling for reshaping round wire stock into lobular form. While this reshaping of the wire stock can be done efficiently, the operation must still be carried out, which necessarily increases the cost of production.
Accordingly, it is been proposed to roll thread a blank of circular cross section to form a lobular-threaded screw. The reshaping of the blank into lobular form takes place during the roll-threading operation. One such proposal is shown in U.S. Pat. No. 3,772,720 to Yamamoto. In that patent there is disclosed a method and apparatus in which the dies are formed with transverse grooves which impart to the blank a unique movement during the roll threading, thereby causing the circular blank to be reshaped into a threaded fastener with an odd number of lobes. Another such proposal is made in U.S. Pat. No. 2,352,982 to Tomalis. In that patent there is a general suggestion that the thread is formed on a cylindrical screw blank by the use of dies whose surfaces are not plain but may be considered as being scalloped and grooved for uniform depth along the scallops. Each scallop is to have a chordal length along the die equivalent to one face of the finished screw blank. The die structure in the Tomalis patent is not otherwise disclosed and its construction is unclear and uncertain.